INTRODUCTION:

Cotton fiber is mostly used in textile industries and provides numerous advantages and comfort in everyday life. A major drawback of cotton fiber is its inherent tendency to burn and associated fire hazards. The fire risk due to high flammability of cotton textile remains always with us because of the high specific surface area of textiles, which facilitate maximum contact to atmospheric oxygen. Therefore, fire retarding treatments of natural occurring textiles has attracted much attention from the view point of fire hazard prevention. The public demand for increased safety from fire hazards has led to greater interest in flame retardant (FR) materials^1,2. Many approaches have been developed to impart flame retardant properties to textiles such as pad-dry-cure³, fire retardant additives^4,5, intumescent fire retardant systems^6,7, graft polymerization^8,9and chemical treatments including our studies^10-14. Thermal degradation behaviour of cellulose has been studied by many workers^15,16.

Many fire retardant additives are found effective in decreasing the flammability but cause increase in smoke production¹⁷. The development of environmentally friendly and durable flame-retardant treatment for cotton is still a challenge.

Flame retardant compounds change the path of decomposition of cotton and prevent the formation of laevoglucosan and other flammable volatiles¹⁸.The function of a flame retardant is to increase char at the cost of flammable volatiles. Intumescent coating is one of the methods used to impart flame retardancy to textiles. The intumescent system is a reactive material whose decomposition processes are extremely dynamic. Intumescent coating behaviour is generally observed differently in different fire conditions, coating thickness and protected structures. Intumescent flame retardant system requires acid source, a swelling agent and a carbonizing agent^19,20. Many flame retardants for cotton fabrics have been developed as coating formulations without taking into account the toxic effect of flame retardants. This study presents improvement in flame retardant property by selecting environment friendly phosphorus based intumescent system containing nanoclays. The main function of an intumescent coating is to protect the underlying material from the heat of fire and avoid the contact of substrate from atmospheric oxygen. It is observed that the addition of nanoclays increase the thermal stability by acting as thermal insulator and mass transport barrier^21,22 to the volatile products generated during decomposition of materials.

The aim of this work is to study systematically the flame retardant effects of nano sized clays such as kaolin, pristine nanoclay (bentonite), and organically modified nanoclays (nanomer 1.34TCN and nanomer 1.28E) present in intumescent formulation consisting ammonium polyphosphate as an acid source, melamine as a swelling agent and pentaerythritol as a carbon source on cotton fabric in terms of thermal behaviour, synergy and their influence on char formation. Different flame retardant formulations containing nanoclays for cotton were investigated using thermogravimetry (TG) and differential scanning calorimetry (DSC) techniques. The flammability behavior was studied using auto flammability and limiting oxygen index (LOI) tests.

EXPERIMENTAL:

Materials:

The Cotton fabric used in this study was donated by Raymond Zambaiti Ltd. Kolhapur (warp count =50, weft count =50, GSM=132, ends per cm=68, picks per cm=37) which was already desized, mercerized and bleached cotton. The cotton fabric was hand washed with water for 15 minutes and dried. The materials used for intumescent flame retardant system were ammonium polyphosphate (APP) as Exolit AP 422 donated by Clariant Inc., USA, as an acid source; melamine as a swelling agent and pentaerythritol as a carbon source supplied by CDH Chemicals Co., India. Different nanoclays such as kaolin (KLN), bentonite (BNT), nanomer 1.34TCN (NMT) and nanomer 1.28E (NME) used were obtained from Sigma Aldrich Co., India. Zytrol 25 an acrylic resin was used as binder supplied by Zydex Industries, India. All these materials were procured and used as received.

Preparation of formulations for intumescent coating:

The various coated samples of cotton fabric were prepared using intumescent formulations with different nanoclays. The intumescent formulation (INT) was prepared by making slurry of APP, melamine and pentaerythritol in 3:1:1 ratio and 15 % binder in water as a solvent.

To prepare total 50 g slurry for coating, first of all, the solid content for each component was determined by heating at 100 ⁰C for 1 hr in an oven. Then individual amount (Oven Dry (OD) required) of different components of formulations were calculated according to their percentage composition (parts) taken in formulations and then water content was calculated. Similarly, intumescent formulations containing different nanoclays (6 %) were prepared to see their effects on flame retardancy (Table 1).

Table 1 Components required for intumescent system containing nanoclay.

Components	Parts	Solid content (%)	*OD^required (g)**	Slurry^# (g)
Ammonium polyphosphate	60.00	100.00	9.92	9.92
Melamine	20.00	100.00	3.31	3.31
Pentaerythritol	20.00	100.00	3.31	3.31
Binder	15.00	26.00	2.48	9.53
Bentonite clay	6.00	87.00	0.99	1.14
Total	121.00	--	20.00	27.21
Water				22.79
Target solid				40.00
Total amount				50.00

^*OD (oven dry) required = (parts/total) × (target solid × amount of slurry)/100

^#Slurry = (OD required/solid content) × 100

Method of fabric coating with K-control coater:

The intumescent coatings were done using an automatic bar coater (RK-Print Coat Instrumrnt Ltd. UK, model K 101) by spreading the coating paste on cotton fabric with a rod coater. The rods bearing different number are generally used to obtain different coating thickness. In this study, the rod coater bearing No. 1 was used in order to obtain an expected coating of about 35 % add on weight. The prepared slurry was spread out on the fabric of 20 cm ×30 cm size with the help of chosen rod coater. The coated fabric samples were immediately placed in an oven to dry for 2 min at 105 ⁰C in order to eliminate the residual water and to crosslink the coating layer. Then add on weight was determined and it was found 35 %.

Thickness measurement:

The thickness of fabric was measured by the Prolific Thickness Tester instrument (BS 2544:154). The thickness gauge was used to measure the thickness of pure cotton fabric and coated fabric samples. Thickness at different places on sample was measured and the mean was calculated.

Stiffness measurement:

The resistance of the fabric to stiffness was measured using Paramount Stiffness Tester (BS 3356:1961). Test specimens measuring 2.5 cm x 12 cm were cut in both warp and weft directions from different portions of the fabrics. The test was repeated for all the samples and an average was calculated.

Thermal analysis

The Differential scanning calorimetry (DSC) and Thermogravimetry (TG) analyses were carried out on a NETZSCH STA 449F1 TG instrument under static air from ambient temperature to 700 ⁰C at a heating rate of 10 K min^-1for pure and coated samples.

Flammability test

The burning behaviour of pure and coated cotton samples were studied by using the ATLAS 45⁰ Automatic Flammability Tester according to ASTM D1230. The ignition time was set 12 s. The flammability tests were carried out using cotton fabric sample of 15 cm x 6 cm size.

Limiting oxygen index (LOI) values were also measured using Limiting Oxygen Indexer IS: 13501-1992 RA 2008 instrument. Pure and coated samples of size 150 mm×50 mm were taken for LOI test.

RESULTS AND DISCUSSION:

Thermal analysis

DSC study: DSC curves of samples were obtained upto 700 ⁰C in air atmosphere and are shown in Figures 1 and 2. The initiation and maximum temperatures of various DSC peaks in air atmosphere were measured and are given in Table 2.

DSC curve of pure cotton fabric (CF) shows two major exothermic peaks with maxima at 350 ⁰C and 472 ⁰C, respectively. First exotherm with maximum at 350 ⁰C is large, which may be due to dehydration (charring) and oxidation of the products (laevoglucosan which is a major volatile product) of thermal depolymerization of cellulose. The second exotherm with maximum at 472 ⁰C may be due to the oxidation of charred residue formed²³.

DSC curve of CF–INT shows the first exotherm (small) at 298 ⁰C and second exotherm (large) with maxima at 338 ⁰C, which are lowered in comparison to pure cotton cellulose due to chemical interactions among coated additives and with cotton cellulose such as catalyzed dehydration of cellulose, phosphorylation of cellulose and PER, cross–linking of APP and oxidation of products of thermal decomposition of cellulose. APP and melamine components of intumescent system on decomposition release phosphoric acid and ammonia, respectively.

The third exotherm in DSC curve of CF–INT, with maximum at 490 ⁰C may be due to cross-linking, deoxygenation and aromatization reactions of the char residue²⁴ formed in air atmosphere. The shift of last exotherm of CF at 472 ⁰C to higher temperature at 490 ⁰C in case of CF-INT is an indication of increase in thermally stability of material, which may be attributed to the formation of carbonaceous layer generally called char at the surface of fabric. The increase in char of cotton fabric on coating with intumescent system is also observed in TG analysis (Table 3). The carbonaceous layer reduces the heat and mass transfer between the substrate and the flame. As a result of this, it insulates the cotton fabric from flame and atmospheric oxygen as indicated by reduction to greater extent of the size of peak (Figure1). The acrylic based binder used may also act as a carbon source in the intumescent system, which has been demonstrated in earliest study²⁵. The area under oxidative exothermic peaks are substantially decreased after intumescent coating, which indicates flame reducing effects by intumescent and decreasing the oxidation of volatile products by preventing the contact with atmospheric oxygen.

On incorporating the nanoclays into intumescent system, no significant changes are observed in DSC curves except that the last exotherms maxima are slightly shifted to lower temperatures because of catalytic activity of nanoclays being having the large surface area. The CF–INT–NMT sample shows very small first exotherm, and CF–INT–NME sample does not give the first exotherm, which may be due to overlapping of this exotherm with next exotherm.

Figure 1 DSC curve of pure and coated cotton fabric samples.

Table 2 Characteristic values of DSC measurements in air atmosphere

Sample	Sample abbreviation	DSC temperature (⁰C)		Nature of peak
Sample	Sample abbreviation	Initiation	Maximum	Nature of peak
Pure cotton fabric	CF	337 445	350 472	Exo (large and sharp) Exo (large and sharp)
Cotton fabric coated with intumescent	CF-INT	274 317 435	298 338 490	Exo (small and sharp) Exo (small and broad) Exo (small and broad)
Cotton fabric coated with intumescent containing Kaolin	CF-INT-KLN	272 318 420	300 334 482	Exo (small and sharp) Exo (small and broad) Exo (small and broad)
Cotton fabric coated with intumescent containing Bentonite	CF-INT-BNT	270 314 420	297 346 485	Exo (small and sharp) Exo (medium) Exo (small and broad)
Cotton fabric coated with intumescent containing Nanomer 1.34TCN	CF-INT-NMT	-- 315 435	305 335 478	Exo (very small) Exo (medium) Exo (small and broad)
Cotton fabric coated with intumescent containing Nanomer 1.28E	CF-INT-NME	-- 315 440	-- 341 480	-- Exo (medium) Exo (small and broad)

Figure 2 DSC curves of coated cotton fabric samples.

Thermogravimetric analysis

TG curves of pure cotton and coated fabric samples are shown in Figures 3 and 4. The parameters used for comparing thermal stability are T_10wt%, (temperature at 10 % mass loss), T_50wt% (temperature at 50 % mass loss) and the residual mass i.e. char at 600 ⁰C and are given in Table 3.

Pure cotton fabric (CF) shows two stages of thermal degradation with major weight loss of 78.0 % in the first stage (100-360 ⁰C) with DTG peak at 330 ⁰C. The onset temperature of degradation (the temperature at which 10 % wt. loss takes place, T_10wt%)) and T_50wt%(the temperature at which 50 % wt. loss takes place) of pure cotton fabric are 313 and 330 ⁰C, respectively. The second stage of thermal degradation in the temperature range 360-510 ⁰C with DTG peak at 470 ⁰C loses 20.5 % weight. The cotton fabric degrades almost completely upto 500 ⁰C leaving no char yield at 600 ⁰C. The major weight loss of pure cotton is due to dehydration, decomposition and formation and evaporation of volatile products mainly laevoglucosan²⁶. Later, in second stage of thermal degradation, the oxidation of carbonaceous residue formed earlier in first stage of thermal degradation takes place. Thermal degradation of APP has already been studied which eliminates ammonia and water in first step leading to the highly cross-linked polyphosphoric acid. The second step corresponds the polyphosphoric acid evaporation and/or dehydration to P₄O₁₀which later sublimes.

TG curve of CF-INT shows different behaviour with three stages of the degradation with char yield of 26.8 % at 600 ⁰C. After coating with intumescent, the onset temperature of CF-INT sample is decreased by 44 ⁰C due to acid catalyzed dehydration by phosphoric acid released from APP. But the temperature at mid-point of decomposition (T_50wt%) is increased by about 54 ⁰C due to formation of protective carbonaceous layer on the surface of cotton fabric and cross-linked polyphosphate structure of intumescent material. The degradation rate of CF-INT sample is decreased and delayed by limiting the heat and mass transfer. The increase in thermal stability is observed in higher temperature range i.e. after 340 ⁰C as indicated by formation of higher amount of char (22.7 %) at 600 ⁰C. In

the intumescent system, degraded material at surface protects the substrate from oxidation and flame. At higher temperature (600-700 ⁰C), a 13 % weight loss of CF-INT is observed, which is probably due to oxidation of aromatic charred residues.

On incoprating nanoclays into intumescent system, no significant change in thermal behaviour is observed. The weight loss in second stage is found increased in the temperature range 310-600 ⁰C and slightly reduced in third stages of degradation in the temperature 600-700 ⁰C. The T_50wt% temperature is reduced in all samples containing nanoclays due to catalytic action of the nanoclays and also supported by decrease in DTG peaks. The T_50%wt temperature is highly decreased in case of sample containing NMT. Onset temperature of samples containing clays slightly increases in comparison CF-INT. The char yields formed are slightly decreased (22.7 %) at 600 ⁰C, which may be due to catalytic effects of clay on degradation process of polymer due to their large surface area.

At 500 ⁰C, pure cotton fabric gives negligible char but after coating treatment with intumescent, about 37% residue is left at this temperature due to interaction amongst intumescent components and interaction of intumescent components with cotton cellulose. The char layer insulates the underlying cotton material and reduces the volatile products²⁸ thereby increasing the thermal stability of coated cotton at higher temperature. There is a strong correlation between char yield and fire resistance as the char is formed at the expense of combustible gases and the presence of a char inhibits further flame spread by acting as a thermal barrier around the unburned material.

Figure 3 TG curves of pure and coated cotton fabric samples.

Table 3 TG data of pure and coated cotton fabric samples in air atmosphere

Sample	Stages	Temp. range (⁰C)	Weight loss (%)	DTG (⁰C)	T_10wt% (⁰C)	T_50wt%(⁰C)	Char at 600 ⁰C (%)
CF	1^st 2^nd	100-360 360-510	78.0 20.5	330 470	313	331	0.35
CF-INT	1^st 2^nd 3^rd	100-310 310-600 600-700	40.0 32.5 13.3	286	269	385	26.8
CF-INT-KLN	1^st 2^nd 3^rd	100-310 310-600 600-700	39.6 37.0 9.0	290	276	378	22.7
CF-INT-BNT	1^st 2^nd 3^rd	100-310 310-600 600-700	40.4 37.4 7.4	284	270	372	21.5
CF-INT-NMT	1^st 2^nd 3^rd	100-310 310-600 600-700	42.0 37.2 8.5	281	274	359	20.2
CF-INT-NME	1^st 2^nd 3^rd	100-310 310-600 600-700	40.8 36.6 9.2	281	274	370	22.0

Table 4 Stiffness, thickness and flammability parameters of pure and coated cotton fabric samples.

Sample	Stiffness		Thickness (mm)	Auto flammability test		LOI (%)
Sample	Warpwise (cm)	Weftwise (cm)	Thickness (mm)	Flame spread time (sec)	Char length (cm)	LOI (%)
CF	3.2	2.6	2.2	13	BEL^#	18.0
CF-INT	6.2	5.9	2.9	DNI^*	1.8	27.5
CF-INT-KLN	6.2	4.4	2.8	DNI	1.3	27.5
CF-INT-BNT	6.2	4.0	2.7	DNI	1.6	28.0
CF-INT-NMT	6.4	4.9	2.8	DNI	1.6	28.0
CF-INT-NME	6.4	5.0	2.8	DNI	2.3	28.5

^*DNI-Did Not Ignite, ^#BEL-Burn Entire Length

Thickness and Stiffness measurement

Thickness and stiffness of pure and coated cotton fabrics/samples were measured and given in Table 4. The thickness of pure cotton fabric (CF) was observed 2.2 mm as compared to intumescent coated cotton fabric (CF-INT) of thickness 2.9 mm and thickness of cotton fabric coated with both intumescent and nanoclays varies from 2.7 to 2.8 mm.

The stiffness in warp wise direction of pure cotton fabric was 3.2 cm but when cotton fabric coated with intumescent the stiffness was observed as 6.2 cm and it remains almost same for the cotton fabric coated with intumescent containing nanoclays. In case of weft wise direction, stiffness was observed 2.6 cm for pure cotton fabric and increased to 5.9 cm for intumescent coated cotton fabric. On addition of nanoclays with intumescent stiffness starts to decrease. Thickness and stiffness of coated cotton fabric samples are not too high than untreated cotton fabric which indicates that the properties of cotton fabric would have not been affected.

CONCLUSION

In this study, the cotton fabric was coated with flame retardant intumescent formulations containing different nanoclays to impart flame resistance to cotton fabric. The thermal analysis of treated cotton fabric shows decrease in the onset temperature and increase in mid-point temperature of degradation. Intumescent system limits the destabilization and increase the thermal stability after 340 ⁰C leading to the higher char yields. No significant change in thermal behaviour of cotton fabric is observed on inclusion of nanoclays. LOI value for pure cotton fabric (18 %) is found increased to 27.5 % for coated cotton fabric with intumescent. LOI value is further increased slightly

up to 28.5 % on inclusion of nanoclays in intumescent formulation. The flammability tests (Auto flammability and LOI) indicate that the intumescent coating has provided good flame retardancy to cotton fabric.

ACKNOWLEDGEMENT:

Authors acknowledge Mr Amit Sharma, Clariant Inc., for providing the flame retardant material for this study.

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Received on 01.06.2013 Modified on 20.06.2013

Asian J. Research Chem. 6(7): July 2013; Page 676-682